1. Technical Field
The present invention relates generally to a method and a device for adjusting power amplifier properties. Particularly the invention concerns techniques exploiting PAR (Peak to Average Ratio) in the adjustment.
2. Discussion of Related Art
Many traditional communication systems utilize modulation formats, for example GSMK (Gaussian Minimum Shift Keying) in the case of GSM (Global System for Mobile communications), wherein amplitude envelope stays constant resulting in low, in principle zero, PAR values making possible the use of affordable and non-linear amplifiers. As a generalization, if the PAR value is high, which is due to the large variance of the signal amplitude, amplifier circuit linearity must be improved to achieve low enough ACLR (Adjacent Channel Leakage power Ratio) values. Therefore, an amplifier must have enough amplification headroom, so called “back-off”, to avoid distortions in an output signal. The distortions in the signal include e.g. second- and third-order harmonics which leak into the adjacent channels. This leakage is identical to the intermodulation caused by two sinusoidal signals input to an amplifier. Moreover, high PAR values are undesirable as they place considerable requirements for the dynamic range of A/D and D/A converters without forgetting large power dissipation in the highly linear and large back-off amplifiers. The circuit linearity can be measured as a back-off number from the amplifier input compression point or some other specific point in the radio chain.
FIG. 1 discloses Pinput-Poutput characteristics of a power amplifier, wherein active power ranges of two signals, one of signal 102 with higher PAR value PARh and another of signal 104 with lower PAR value PAR1, both signals having a common average power Pavg, are illustrated with dotted lines. As indicated by the figure, the amplifier has to be designed according to maximum possible Pavg and PAR values, which is not optimal whenever the actual operational range is something less demanding. The reduction in efficiency resulting from too high back-off can be estimated by the following equation, which defines the efficiency η of the class-B amplifier as a function of the output power Poutputη=ηmax√{square root over (Poutput/Pmax)}wherein Pmax is the maximum output power and ηmax is the efficiency at Pmax. Considering an ideal class-B amplifier with a maximum efficiency of π/4 and applying Pmax/Pavg (PAR) value of 3.2 dB, being typical PAR for WCDMA (Wideband Code Division Multiple Access) PAs, and 5.5 dB, respectively common for HSDPA (High Speed Downlink Packet Access) PAs, efficiencies of 54.3% and 41.7% are achieved. Thus, the efficiency of the WCDMA PA is reduced by the factor of 0.768 if the HDSPA PA is utilized for both purposes.
In WCDMA and multicarrier, e.g. OFDM (Orthogonal Frequency Division Multiplexing), systems amplifier design and control is particularly relevant issue as the modulation techniques thereof are non-constant in order to achieve higher data rates than constant techniques like GMSK can provide. The fundamental idea behind the WCDMA is to allow several users to share the same spectrum by spreading the data across the available frequency band. This can be achieved if the user's data signal is multiplied with a pseudorandom “chip” sequence of a higher bit rate to fill the frequency band. In the receiving end the single user's data can be retrieved from the overall received signal by cross-correlating the received signal with a synchronized copy of the chip signal. The procedure is called dispreading. Naturally, chip sequences should be designed to be orthogonal so as to be maximally separable in the reception sense. On the other hand, OFDM is a multicarrier system wherein the available frequency band includes several narrowband carriers with overlapping sidebands. If each carrier's spectrum is ideally shaped like a sinc function having zero crossings every f0, and the carrier separation is f0 as well, the resulting overall spectrum is spectrally efficient (dense), and at the same time, the co-channel interference is retained to be low. In practice, OFDM transceivers convert a data signal to PSK (Phase Shift Keying) or QAM (Quadrature Amplitude Modulation) symbols after which the serial stream is converted to a parallel form, modulated with inverse FFT (Fast Fourier Transform), serialized and finally converted to analog for transmission. The receiver executes the aforesaid process in reverse order utilizing e.g. FFT.
The difference between the average and peak power of the signal, which can be presented via PAR, results from the fact that multiple carriers may add together either constructively to form a high level signal or destructively to form a very low level signal. Therefore, as the number of sub-carriers is increased, also the PAR inevitably increases. The same phenomenon occurs in the WCDMA system as the PAR typically increases with the number and types of codes transmitted. A WCDMA system supports multirate data transfer in two major ways, namely by using a variable spreading factor and by utilizing a multicode approach, wherein higher data rates are achieved via use of several parallel codes. In addition, forthcoming HSDPA based on a HS-DSCH (High Speed Downlink Shared Channel) is a packet data service in WCDMA downlink to transmit data up to 20 Mbps (MIMO, Multiple Input Multiple Output, systems with multiple transmitter and receiver antennas). It features a concept of AMC (Adaptive Modulation and Coding) enabling utilization of adaptive modulation and coding techniques in order to maximize the data throughput in changing channel conditions. The channel conditions can be estimated, for example, based on feedback from the receiver and use AMC instead of fast power control to compensate for the variations in the channel estimate. Users close to the base station may be assigned higher order modulation with higher code rates (e.g. 64 QAM and R=¾ turbo codes), but the lower modulation-order, e.g. QPSK, and code rate will be taken into use as the distance from the base station increases and/or the degradation in the channel conditions is detected.
It's obvious that with an adaptive system like AMC in WCDMA HSDPA, PAR values may be high as the system intermittently changes transmission parameters affecting also the amplitude envelope and extends its possible range. It's admitted that many solutions have been proposed for minimizing PAR, see, for example, U.S. Pat. Nos. 6,294,956, 6,292,054 and 5,894,498 for reference. However, despite the existing solutions, PAR fluctuations, which normally originate from the changes in the utilized transmission techniques with possibly varying modulation types and/or number of multicodes, cannot be completely removed. Also, in the case of multicode transmission, PAR of the amplified signal may vary based on gain factors i.e. power offsets between different codes. This is the case especially in HSDPA (High Speed Downlink Packet Access) service. These gain factors between codes are signalled by higher layers in the network. As presented above, in the worst case the PAR in HSDPA uplink signal can even be ˜2.3 dB higher compared to the standard WCDMA scenario. Recalling that the PAR value directly affects to the power consumption of a PA and considering, for example, mobile terminals' quite limited power supply in a form of compact and relatively low capacity batteries, it surely isn't a surprise that power amplifiers are one of the most power hungry building blocks in a terminal as their output power back-off is typically designed according to the worst case PAR and Pavg values possible in the system.